Three lifeboats were deployed, but the 17 passengers and two crew had already been picked up from their liferaft by a rescue craft launched from the nearby Nord Nightingale vessel and taken back to the tanker, the coastguard confirmed.

From there, helicopters took the rescued workers to Kirkwall on the Orkney islands.

A statement from CHC said: "CHC Helicopter can confirm that all 17 passengers and two crew have been picked up by the standby vessel Nord Nightingale following the controlled ditching of one of its EC225 Super Puma aircraft.

"All 19 people on board are safe and well.

"The appropriate authorities have been informed and a full investigation will be undertaken to determine the cause of the incident."

The rig is owned by French oil giant Total.

Scotland's First Minister Alex Salmond called for a swift investigation.

"It will be a huge relief to all of the friends, family and colleagues of Scotland's offshore workers that each one of the 19 people aboard the helicopter that ditched in the North Sea this afternoon have now been accounted for and are safe and well," he said.

"Once again, the response to this incident by our emergency services was fantastic and their continued bravery and expertise will bring great reassurance to every offshore worker who is required to use helicopters in challenging conditions regularly throughout their working lives."

It is the latest in a series of incidents involving helicopters in the North Sea.

In May all 14 people on board a Super Puma helicopter were rescued after it ditched off the coast of Aberdeen.

Sixteen men died when a Super Puma helicopter plunged into the sea when its gearbox failed as it was flying from BP's Miller platform to Aberdeen in April 2009.